Compositions based on olefin polymers, in particular crystalline polypropylene, filled with wood flour, are already known; because of their low cost and low specific weight (about 1 g/cm.sup.3), these compositions manufactured in the form of thermoformed sheets, found good application possibilities in the automotive, appliance, and building industries. However, some of the physical-mechanical properties of said compositions are poor or barely satisfactory, such as in the case of dimensional stability at temperatures higher than ambient temperature, 40.degree.-70.degree. C. for example, which are usually required for the above mentioned applications, even for prolonged periods. Moreover, articles comprising said compositions can be obtained only by way of sheet thermoforming (previously obtained by using flat-die extruders) operating under very mild conditions (in particular at temperatures lower than 200.degree. C.). With this technique, however, it is impossible to obtain articles with a complex form at fast molding cycles and without waste (scrap). For this purpose, one should resort to injection molding techniques with the proper molds, but said techniques would require, for the above mentioned composites, considerably more drastic operating conditions, in particular temperatures higher than 200.degree. C., and generally ranging from 220.degree. to 230.degree. C., where the cellulosic phase would be subjected to significant thermodegradation, and consequently the articles would have an objectionable smell, poor appearance, and poor mechanical properties; moreover, the molding equipment and molds would be prone to damaging corrosion phenomena.